Re: [Haskell-cafe] How to understand the 'forall' ?

I think I've understood the existential types thing, but I still can't put them to work when I think to a solution for a particular problem, i.e. it's not among my programming tools yet.

Here's another commonly found, easy-to-understand example that's useful.

{-# LANGUAGE ExistentialQuantification #-}

module Main where

data Showable = forall a . Show a => S a

instance Show Showable where show (S x) = show x

main = print [S 1, S 'a', S 9.4, S (Just 2)] -- prints [1,'a',9.4,Just 2]

The datatype says that for any type 'a' with an instance of 'Show a', we can construct a value of 'Showable'. While we normally wouldn't be able to build a list with different types, we can now create a [Showable] list with existential types. Sean